Interaction of Alkaline Earth Cations with the Negatively Charged Phospholipid 1,2-Dimyristoyl-sn-glycero-3-phosphoglycerol: A Differential Scanning and Isothermal Titration Calorimetric Study

Abstract

The interaction of the alkaline earth cations Mg2+, Ca2+, and Sr2+ with 1,2-dimyristoyl-sn-glycero-3-phosphoglycerol (DMPG), dispersed in 0.1 M NaCl at pH 7, was investigated by differential scanning calorimetry (DSC) and isothermal titration calorimetry (ITC). Both calorimetric methods revealed that the interaction of Mg2+ and Sr2+ with DMPG is of similar nature but different from the behavior of Ca2+. In the first heating scans of DMPG:Mg2+ and DMPG:Sr2+ complexes, transition peaks at 31−35 and 60 °C are observed. The higher transition peak disappears in consecutive heating and cooling cycles and only the transition at 31−35 °C remains. The transition at lower temperature occurs from a metastable gel phase into a liquid−crystalline phase, whereas the high-temperature transition occurs from the stable quasicrystalline lamellar phase. The thermotropic behavior of DMPC:Ca2+ complexes is different. The high-temperature transition from the stable state is centered at 80 °C. Cooling induces a metastable gel p...